(Not Applicable)
(Not Applicable)
The present invention generally relates to non-destructive inspection technology and more particularly to a device that utilizes eddy currents to detect cracks, flaws and other defects in materials.
Various instruments for flaw detection in structures have been developed that rely upon the generation of eddy currents in the body of the structure being tested. Flaws in the metallic structures are detected by their perturbation of the eddy currents. The eddy currents are generated in the metallic structure by positioning the structure within an alternating magnetic field. Perturbation of the eddy currents because of a localized presence of a defect in the metallic structure creates a resultant change in the magnetic flux associated with these eddy currents. When such change is detected, it is an indication of the presence of defects in the structure. Such use of eddy currents has been practiced for detecting flaws in solid metallic slabs, metallic pipes, and in layered metallic structures such as the outside surfaces of aircraft.
A single coil may be utilized in the eddy current test probe. The single coil induces a magnetic field in the structure being tested. Flaws are detected by noting changes in the voltage and phase angle of the signal applied to the test coil. Alternatively, a drive coil and a sense coil may be used for the eddy current test probe. Detection of flaws in the test structure is achieved by utilizing voltage variations in the sense coil. Typically, the voltage variations are detected utilizing a null bridge.
Detection of flaws utilizing eddy currents is accomplished by comparing the phase/voltage variations of the test coil to a known response for a particular defect.
Accordingly, the phase/voltage response of the test coil can be compared to a table of responses to determine the size and nature of the defect. The table of responses is generated by testing known defects with the eddy current test probe and detecting the phase/voltage response for such known defect.
Often times an inspection effort is difficult to accomplish due to the geometry, manufacture, service, interference, re-work and occasional abuse of the area. Any one of these factors or a combination thereof may require reaming and/or flex-honing of the inspection area to make such suitable. Additionally, false defect indications are sometimes encountered when metal chips are embedded between layers of stacked structures that form the structure. These chips must be removed mechanically before an accurate inspection effort can be undertaken. Accordingly, there is currently a need for an eddy current inspection probe which can accurately detect defects within structures in varying conditions.
Prior art eddy current probes do not provide consistent responses for defects smaller than 0.030xe2x80x3. The depth-length aspect ratio for such cracks creates uncertainty of the phase/voltage response with conventional eddy current probes. The coils of prior art eddy current probes can typically detect small and large defects in a single frequency of the eddy current field. However, the magnetic field induced within the test structure is broad and less intense at the extremities of the defect such that the phase/voltage responses for the extremities of the defect vary greatly. Additionally, phase/voltage responses for such defects may vary over a range of frequencies applied to the coil of the eddy current probe.
The present invention addresses the above-mentioned deficiencies in prior art eddy current probes by providing a probe that is accurate over a broad range of frequencies. Accordingly, phase/voltage responses obtained for multiple frequencies generated by the eddy current probe can be compared to achieve greater accuracy of defect measurement. Additionally, the present invention provides for a focused eddy current measurement method such that an enhanced crack profile aspect ratio of the defect can be determined.
In accordance with the present invention there is provided an eddy current probe for the detection of defects within a structure. The probe has a sensing end with a coil disposed therein. The coil is operative to generate an eddy current field within the structure when placed adjacent to the structure. Furthermore, the probe of the present invention includes a metallic shield disposed around the coil and operative to focus the eddy current field within the structure. In order to generate the eddy current field, the coil is in electrical communication with a frequency generator. The frequency generator is operative to generate a high and low frequency signal within the coil in order to enhance detection of defects within the structure.
In accordance with the present invention, the shield may be fabricated from a Mu metallic material that is ferromagnetic. The frequency generator typically produces a high frequency signal of approximately 1 MHz and a low frequency signal of approximately 150 KHz. In electrical communication with the coil is a voltage and phase sensor operative to determine the voltage and phase angle of the high and low frequency signals. The frequency and phase responses are indicative of the size of the defect in the structure such that an engineering analysis of the defect can be made. The high frequency signal determines the length of the defect whereas the low frequency signal determines the depth of the defect.
In the preferred embodiment of the present invention, the coil has a generally circular configuration and the shield is generally cylindrical. As such, the shield surrounds coil and focuses the direction of the eddy current field. The sensing end of the probe includes a slit in order to vary the diameter of the probe such that the probe may be insertable within a hollow structure. Typically, the sensing end includes a longitudinal axis and the coil is in generally spaced parallel relation to such axis. The sensing end may be generally spherical and attached to a generally cylindrical shaft in order to insert the probe within the interior of the structure to be analyzed.
In accordance with the preferred embodiment of the present invention, there is provided a method of determining a size of a defect within a hollow structure using an eddy current probe having a coil shielded by a Mu metallic material. The method comprises the steps of inserting the probe within the interior of the structure and generating a high and low frequency signal within the coil. Accordingly, an eddy field is produced within the structure and focused by the Mu shield. The phase angle and voltage response of the high and low frequency signals are detected to determine the size of the defect.
The high and low frequency signals are generated by a frequency generator and applied to the coil simultaneously and analyzed separately in order to determine the size of the defect. In order to fully analyze the defect, the probe is rotated within the hollow interior of the structure in order to scan the entire inside surface thereof. A voltage and phase angle analyzer detects the voltage and phase angle of the high and low frequencies in order to determine a size of the defect. Typically the high frequency signal is analyzed to determine the length of the defect and the low frequency signal is analyzed to determine the depth of the defect.